A 19-year-old man has launched into a rambling outburst on his first appearance in court charged with the murders of his 48-year-old mother, her 15-year-old daughter and eight-year-old son in Perth's north-eastern suburbs.

Teancum Vernon Petersen-Crofts was chargedafter his mother, Michelle Petersen, and her son, Rua, were found dead and her daughter Bella was found critically injured at a house in Ellenbrook, about 30 kilometres north-east of the city centre, in the early hours of Sunday morning.

The teenager, who according to police also lived at the house on Brixton Crescent — which government records say is a state housing property — is expected to appear again in Stirling Gardens Magistrate Court next week.

Community in shock over 'tragedy'

Aliyah Grey and Hunter Turinui-Kaire, two friends of the teenage girl, said the news was hard to take in.